Asymmetrical tin-coupled rubbery polymers, and method of making

ABSTRACT

Tire rubbers which are prepared by anionic polymerization are frequently coupled with a suitable coupling agent, such as a tin halide, to improve desired properties. It has been unexpectedly found that greatly improved properties for tire rubbers, such as lower hysteresis, can be attained by asymmetrically coupling the rubber. This invention more specifically discloses an asymmetrical tin-coupled rubbery polymer which is particularly valuable for use in manufacturing tire tread compounds, said asymmetrical tin-coupled rubbery polymer being comprised of a tin atom having at least three polydiene arms covalently bonded thereto, wherein at least one of said polydiene arms has a number average molecular weight of less than about 40,000, wherein at least one of said polydiene arms has a number average molecular weight of at least about 80,000, and wherein the ratio of the weight average molecular weight to the number average molecular weight of the asymmetrical tin-coupled rubbery polymer is within the range of about 2 to about 2.5.

This application is a divisional of Ser. No. 09/008,716 filed Jan. 19,1998 now U.S. Pat. No. 6,043,321; which claims benefit to ProvisionalApplication 60/037,929 filed Feb. 14, 1997.

BACKGROUND OF THE INVENTION

Tin-coupled polymers are known to provide desirable properties, such asimproved treadwear and reduced rolling resistance, when used in tiretread rubbers. Such tin-coupled rubbery polymers are typically made bycoupling the rubbery polymer with a tin coupling agent at or near theend of the polymerization used in synthesizing the rubbery polymer. Inthe coupling process, live polymer chain ends react with the tincoupling agent thereby coupling the polymer. For instance, up to fourlive chain ends can react with tin tetrahalides, such as tintetrachloride, thereby coupling the polymer chains together.

The coupling efficiency of the tin coupling agent is dependant on manyfactors, such as the quantity of live chain ends available for couplingand the quantity and type of polar modifier, if any, employed in thepolymerization. For instance, tin coupling agents are generally not aseffective in the presence of polar modifiers. The amount of couplingwhich is attained is also, of course, highly dependent upon the quantityof tin coupling agent employed.

Each tin tetrahalide molecule is capable of reacting with up to fourlive polymer chain ends. However, since perfect stoichiometry isdifficult to attain, some of the tin halide molecules often react withless than four live polymer chain ends. For instance, if more than astoichiometric amount of the tin halide coupling agent is employed, thenthere will be an insufficient quantity of live polymer chain ends tototally react with the tin halide molecules on a four to one basis. Onthe other hand, if less than a stoichiometric amount of the tin halidecoupling agent is added, then there will be an excess of live polymerchain ends and some of the live chain ends will not be coupled.

Conventional tin coupling results in the formation of a coupled polymerwhich is essentially symmetrical. In other words, all of the polymerarms on the coupled polymer are of essentially the same chain length.All of the polymer arms in such conventional tin-coupled polymers areaccordingly of essentially the same molecular weight. This results insuch conventional tin-coupled polymers having a low polydispersity. Forinstance, conventional tin-coupled polymers normally having a ratio ofweight average molecular weight to number average molecular weight whichis within the range of about 1.01 to about 1.1.

SUMMARY OF THE INVENTION

This invention is based upon the unexpected finding that greatlyimproved properties for tire rubbers, such as lower hysteresis, can beattained by asymmetrically coupling the rubber. For instance, suchasymmetrically coupled polymers can be utilized in making tires havinggreatly improved rolling resistance without sacrificing other tireproperties. These improved properties are due in part to betterinteraction and compatibility with carbon black. The asymmetrical tincoupling also normally leads to improve the cold flow characteristics ofthe rubbery polymer. Tin coupling in general also leads to betterprocessability and other beneficial properties.

The asymmetrical tin-coupled rubbery polymers of this invention arecomprised of a tin atom having polydiene arms covalently bonded thereto.At least one of the polydiene arms bonded to the tin atom will be a lownumber molecular weight arm having a number average molecular weight ofless than about 40,000. It is also critical for the asymmetricaltin-coupled rubbery polymer to have at least one high molecular weightpolydiene arm bonded to the tin atom. This high molecular weight armwill have a number average molecular weight which is at least 80,000.The ratio of the weight average molecular weight to the number averagemolecular weight of the asymmetrical tin-coupled rubbery polymers ofthis invention will also be within the range of about 2 to about 2.5.

This invention more specifically discloses an asymmetrical tin-coupledrubbery polymer which is particularly valuable for use in manufacturingtire tread compounds, said asymmetrical tin-coupled rubbery polymerbeing comprised of a tin atom having at least three polydiene armscovalently bonded thereto, wherein at least one of said polydiene armshas a number average molecular weight of less than about 40,000, whereinat least one of said polydiene arms has a number average molecularweight of at least about 80,000, and wherein the ratio of the weightaverage molecular weight to the number average molecular weight of theasymmetrical tin-coupled rubbery polymer is within the range of about 2to about 2.5.

This invention also reveals a process for preparing an asymmetricaltin-coupled rubbery polymer which comprises: (1) continuouslypolymerizing at least one diene monomer to a conversion of at leastabout 90 percent utilizing an anionic initiator to produce a polymercement containing living polydiene rubber chains, wherein some of theliving polydiene rubber chains are low molecular weight polydiene rubberchains having a number average molecular weight of less than about40,000, and wherein some of the living polydiene rubber chains are highmolecular weight polydiene rubber chains having a number averagemolecular weight of greater than about 80,000; and (2) continuouslyadding a tin halide to the polymer cement in a separate reaction vesselto produce the asymmetrically tin-coupled rubbery polymer, wherein saidasymmetrical tin-coupled rubbery polymer has a polydispersity which iswithin the range of about 2 to about 2.5.

The stability of the asymmetrical tin-coupled rubbery polymers of thisinvention can be improved by adding a tertiary chelating amine theretosubsequent to the time at which the tin-coupled rubbery polymer iscoupled. N,N,N′,N′-tetramethylethylenediamine (TMEDA) is arepresentative example of a tertiary chelating amine which is preferredfor utilization in stabilizing the polymers of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Virtually any type of rubbery polymer prepared by anionic polymerizationcan be asymmetrically tin-coupled in accordance with this invention. Therubbery polymers which can be asymmetrically coupled will typically besynthesized by a solution polymerization technique utilizing anorganolithium compound as the initiator. These rubbery polymers willaccordingly normally contain a “living” lithium chain end.

The polymerizations employed in synthesizing the living rubbery polymerswill normally be carried out in a hydrocarbon solvent which can be oneor more aromatic, paraffinic or cycloparaffinic compounds. Thesesolvents will normally contain from 4 to 10 carbon atoms per moleculeand will be liquid under the conditions of the polymerization. Somerepresentative examples of suitable organic solvents include pentane,isooctane, cyclohexane, methylcyclohexane, isohexane, n-heptane,n-octane, n-hexane, benzene, toluene, xylene, ethylbenzene,diethylbenzene, isobutylbenzene, petroleum ether, kerosene, petroleumspirits, petroleum naphtha, and the like, alone or in admixture.

In the solution polymerization, there will normally be from 5 to 30weight percent monomers in the polymerization medium. Suchpolymerization media are, of course, comprised of the organic solventand monomers. In most cases, it will be preferred for the polymerizationmedium to contain from 10 to 25 weight percent monomers. It is generallymore preferred for the polymerization medium to contain 15 to 20 weightpercent monomers.

The rubbery polymers which are asymmetrically coupled in accordance withthis invention can be made by the homopolymerization of a conjugateddiolefin monomer or by the copolymerization of a conjugated diolefinmonomer with a vinyl aromatic monomer. It is, of course, also possibleto make living rubbery polymers which can be asymmetrically tin-coupledby polymerizing a mixture of conjugated diolefin monomers with one ormore ethylenically unsaturated monomers, such as vinyl aromaticmonomers. The conjugated diolefin monomers which can be utilized in thesynthesis of rubbery polymers which can be asymmetrically tin-coupled inaccordance with this invention generally contain from 4 to 12 carbonatoms. Those containing from 4 to 8 carbon atoms are generally preferredfor commercial purposes. For similar reasons, 1,3-butadiene and isopreneare the most commonly utilized conjugated diolefin monomers. Someadditional conjugated diolefin monomers that can be utilized include2,3-dimethyl-1,3-butadiene, piperylene, 3-butyl-1,3-octadiene,2-phenyl-1,3-butadiene, and the like, alone or in admixture.

Some representative examples of ethylenically unsaturated monomers thatcan potentially be synthesized into rubbery polymers which can beasymmetrically tin-coupled in accordance with this invention includealkyl acrylates, such as methyl acrylate, ethyl acrylate, butylacrylate, methyl methacrylate and the like; vinylidene monomers havingone or more terminal CH2═CH— groups; vinyl aromatics such as styrene,α-methylstyrene, bromostyrene, chlorostyrene, fluorostyrene and thelike; α-olefins such as ethylene, propylene, 1-butene and the like;vinyl halides, such as vinylbromide, chloroethane (vinylchloride),vinylfluoride, vinyliodide, 1,2-dibromoethene, 1,1-dichloroethene(vinylidene chloride), 1,2-dichloroethene and the like; vinyl esters,such as vinyl acetate; α,β-olefinically unsaturated nitriles, such asacrylonitrile and methacrylonitrile; α,β-olefinically unsaturatedamides, such as acrylamide, N-methyl acrylamide, N,N-dimethylacrylamide,methacrylamide and the like.

Rubbery polymers which are copolymers of one or more diene monomers withone or more other ethylenically unsaturated monomers will normallycontain from about 50 weight percent to about 99 weight percentconjugated diolefin monomers and from about 1 weight percent to about 50weight percent of the other ethylenically unsaturated monomers inaddition to the conjugated diolefin monomers. For example, copolymers ofconjugated diolefin monomers with vinylaromatic monomers, such asstyrene-butadiene rubbers which contain from 50 to 95 weight percentconjugated diolefin monomers and from 5 to 50 weight percentvinylaromatic monomers, are useful in many applications.

Vinyl aromatic monomers are probably the most important group ofethylenically unsaturated monomers which are commonly incorporated intopolydienes. Such vinyl aromatic monomers are, of course, selected so asto be copolymerizable with the conjugated diolefin monomers beingutilized. Generally, any vinyl aromatic monomer which is known topolymerize with organolithium initiators can be used. Such vinylaromatic monomers typically contain from 8 to 20 carbon atoms. Usually,the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. Themost widely used vinyl aromatic monomer is styrene. Some examples ofvinyl aromatic monomers that can be utilized include styrene,1-vinylnaphthalene, 2-vinylnaphthalene, α-methylstyrene,4-phenylstyrene, 3-methylstyrene and the like.

Some representative examples of rubbery polymers which can beasymmetrically tin-coupled in accordance with this invention includepolybutadiene, polyisoprene, styrene-butadiene rubber (SBR),α-methylstyrene-butadiene rubber, α-methylstyrene-isoprene rubber,styrene-isoprene-butadiene rubber (SIBR), styrene-isoprene rubber (SIR),isoprene-butadiene rubber (IBR), α-methylstyrene-isoprene-butadienerubber and α-methylstyrene-styrene-isoprene-butadiene rubber.

The polymerizations employed in making the rubbery polymer are typicallyinitiated by adding an organolithium initiator to an organicpolymerization medium which contains the monomers. Such polymerizationsare typically carried out utilizing continuous polymerizationtechniques. In such continuous polymerizations, monomers and initiatorare continuously added to the organic polymerization medium with therubbery polymer synthesized being continuously withdrawn. Suchcontinuous polymerizations are typically conducted in a multiple reactorsystem.

The organolithium initiators which can be employed in synthesizingrubbery polymers which can be asymmetrically coupled in accordance withthis invention include the monofunctional and multifunctional typesknown for polymerizing the monomers described herein. Themultifunctional organolithium initiators can be either specificorganolithium compounds or can be multifunctional types which are notnecessarily specific compounds but rather represent reproduciblecompositions of regulable functionality.

The amount of organolithium initiator utilized will vary with themonomers being polymerized and with the molecular weight that is desiredfor the polymer being synthesized. However, as a general rule, from 0.01to 1 phm (parts per 100 parts by weight of monomer) of an organolithiuminitiator will be utilized. In most cases, from 0.01 to 0.1 phm of anorganolithium initiator will be utilized with it being preferred toutilize 0.025 to 0.07 phm of the organolithium initiator.

The choice of initiator can be governed by the degree of branching andthe degree of elasticity desired for the polymer, the nature of thefeedstock and the like. With regard to the feedstock employed as thesource of conjugated diene, for example, the multifunctional initiatortypes generally are preferred when a low concentration diene stream isat least a portion of the feedstock, since some components present inthe unpurified low concentration diene stream may tend to react withcarbon lithium bonds to deactivate initiator activity, thusnecessitating the presence of sufficient lithium functionality in theinitiator so as to override such effects.

The multifunctional initiators which can be used include those preparedby reacting an organomonolithium compounded with a multivinylphosphineor with a multivinylsilane, such a reaction preferably being conductedin an inert diluent such as a hydrocarbon or a mixture of a hydrocarbonand a polar organic compound. The reaction between the multivinylsilaneor multivinylphosphine and the organomonolithium compound can result ina precipitate which can be solubilized, if desired, by adding asolubilizing monomer such as a conjugated diene or monovinyl aromaticcompound, after reaction of the primary components. Alternatively, thereaction can be conducted in the presence of a minor amount of thesolubilizing monomer. The relative amounts of the organomonolithiumcompound and the multivinylsilane or the multivinylphosphine preferablyshould be in the range of about 0.33 to 4 moles of organomonolithiumcompound per mole of vinyl groups present in the multivinylsilane ormultivinylphosphine employed. It should be noted that suchmultifunctional initiators are commonly used as mixtures of compoundsrather than as specific individual compounds.

Exemplary organomonolithium compounds include ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium,n-eicosyllithium, phenyllithium, 2-naphthyllithium,4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium and the like.

Exemplary multivinylsilane compounds include tetravinylsilane,methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane,cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane,(3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane and the like.

Exemplary multivinylphosphine compounds include trivinylphosphine,methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine,cyclooctyldivinylphosphine and the like.

Other multifunctional polymerization initiators can be prepared byutilizing an organomonolithium compound, further together with amultivinylaromatic compound and either a conjugated diene ormonovinylaromatic compound or both. These ingredients can be chargedinitially, usually in the presence of a hydrocarbon or a mixture of ahydrocarbon and a polar organic compound as a diluent. Alternatively, amultifunctional polymerization initiator can be prepared in a two-stepprocess by reacting the organomonolithium compound with a conjugateddiene or monovinyl aromatic compound additive and then adding themultivinyl aromatic compound. Any of the conjugated dienes or monovinylaromatic compounds described can be employed. The ratio of conjugateddiene or monovinyl aromatic compound additive employed preferably shouldbe in the range of about 2 to 15 moles of polymerizable compound permole of organolithium compound. The amount of multivinylaromaticcompound employed preferably should be in the range of about 0.05 to 2moles per mole of organomonolithium compound.

Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene,1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene,1,3-divinylnaphthalene, 1,8-divinylnaphthalene,1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4′-trivinylbiphenyl,m-diisopropenyl benzene, p-diisopropenyl benzene,1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatichydrocarbons containing up to 18 carbon atoms per molecule arepreferred, particularly divinylbenzene as either the ortho, meta or paraisomer and commercial divinylbenzene, which is a mixture of the threeisomers, and other compounds, such as the ethylstyrenes, also is quitesatisfactory.

Other types of multifunctional initiators can be employed such as thoseprepared by contacting a sec- or tert-organomonolithium compound with1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithiumcompound per mole of the 1,3-butadiene, in the absence of added polarmaterial in this instance, with the contacting preferably beingconducted in an inert hydrocarbon diluent, though contacting without thediluent can be employed if desired.

Alternatively, specific organolithium compounds can be employed asinitiators, if desired, in the preparation of polymers in accordancewith the present invention. These can be represented by R(Li)x wherein Rrepresents a hydrocarbyl radical containing from 1 to 20 carbon atoms,and wherein x is an integer of 1 to 4. Exemplary organolithium compoundsare methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium,dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane,1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane,1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4′-dilithiobiphenyl and the like.

The polymerization temperature utilized can vary over a broad range offrom about −20° C. to about 180° C. In most cases, a temperature withinthe range of about 30° C. to about 125° C. will be utilized. It istypically most preferred for the polymerization temperature to be withinthe range of about 60° C. to about 85° C. The pressure used willnormally be sufficient to maintain a substantially liquid phase underthe conditions of the polymerization reaction.

The polymerization is conducted for a length of time sufficient topermit substantially complete polymerization of monomers. In otherwords, the polymerization is normally carried out until high conversionsare attained. The polymerization is then terminated by the continuousaddition of a tin coupling agent. This continuous addition of tincoupling agent is normally done in a reaction zone separate from thezone where the bulk of the polymerization is occurring. In other words,the tin coupling agent will typically be added only after a high degreeof conversion has already been attained. For instance, the tin couplingagent will normally be added only after a monomer conversion of greaterthan about 90 percent has been realized. It will typically be preferredfor the monomer conversion to reach at least about 95 percent before thetin coupling agent is added. As a general rule, it is most preferred forthe monomer conversion to exceed about 98 percent before the tincoupling agent is added. The tin coupling agent will normally be addedin a separate reaction vessel after the desired degree of conversion hasbeen attained. The tin coupling agent can be added in a hydrocarbonsolution, e.g., in cyclohexane, to the polymerization admixture withsuitable mixing for distribution and reaction.

The tin coupling agent will normally be a tin tetrahalide, such as tintetrachloride, tin tetrabromide, tin tetrafluoride or tin tetraiodide.However, tin trihalides can also optionally be used. In cases where tintrihalides are utilized, a coupled polymer having a maximum of threearms results. To induce a higher level of branching, tin tetrahalidesare normally preferred. As a general rule, tin tetrachloride is mostpreferred.

Broadly, and exemplary, a range of about 0.01 to 4.5 milliequivalents oftin coupling agent is employed per 100 grams of the rubbery polymer. Itis normally preferred to utilize about 0.01 to about 1.5milliequivalents of the tin coupling agent per 100 grams of polymer toobtain the desired Mooney viscosity. The larger quantities tend toresult in production of polymers containing terminally reactive groupsor insufficient coupling. One equivalent of tin coupling agent perequivalent of lithium is considered an optimum amount for maximumbranching. For instance, if a tin tetrahalide is used as the couplingagent, one mole of the tin tetrahalide would be utilized per four molesof live lithium ends. In cases where a tin trihalide is used as thecoupling agent, one mole of the tin trihalide will optimally be utilizedfor every three moles of live lithium ends. The tin coupling agent canbe added in a hydrocarbon solution, e.g., in cyclohexane, to thepolymerization admixture in the reactor with suitable mixing fordistribution and reaction.

After the tin coupling has been completed, a tertiary chelating alkyl1,2-ethylene diamine can optionally be added to the polymer cement tostabilize the asymmetrically tin-coupled rubbery polymer. The tertiarychelating amines which can be used are normally chelating alkyl diaminesof the structural formula:

wherein n represents an integer from 1 to about 6, wherein A representsan alkylene group containing from 1 to about 6 carbon atoms and whereinR¹, R², R³ and R⁴ can be the same or different and represent alkylgroups containing from 1 to about 6 carbon atoms. The alkylene group Ais the formula —(—CH₂—)_(m) wherein m is an integer from 1 to about 6.The alkylene group will typically contain from 1 to 4 carbon atoms (mwill be 1 to 4) and will preferably contain 2 carbon atoms. In mostcases, n will be an integer from 1 to about 3 with it being preferredfor n to be 1. It is preferred for R¹, R², R³ and R⁴ to represent alkylgroups which contain from 1 to 3 carbon atoms. In most cases, R¹, R², R³and R⁴ will represent methyl groups.

A sufficient amount of the chelating amine should be added to complexwith any residual tin coupling agent remaining after completion of thecoupling reaction.

In most cases, from about 0.01 phr (parts by weight per 100 parts byweight of dry rubber) to about 2 phr of the chelating alkyl 1,2-ethylenediamine will be added to the polymer cement to stabilize the rubberypolymer. Typically, from about 0.05 phr to about 1 phr of the chelatingalkyl 1,2-ethylene diamine will be added. More typically, from about 0.1phr to about 0.6 phr of the chelating alkyl 1,2-ethylene diamine will beadded to the polymer cement to stabilize the rubbery polymer.

After the polymerization, asymmetrical tin coupling, and optionally thestabilization step, has been completed, the asymmetrical tin-coupledrubbery polymer can be recovered from the organic solvent. Theasymmetrical tin-coupled rubbery polymer can be recovered from theorganic solvent and residue by means such as decantation, filtration,centrification and the like. It is often desirable to precipitate theasymmetrically tin-coupled rubbery polymer from the organic solvent bythe addition of lower alcohols containing from about 1 to about 4 carbonatoms to the polymer solution. Suitable lower alcohols for precipitationof the rubber from the polymer cement include methanol, ethanol,isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. Theutilization of lower alcohols to precipitate the asymmetricallytin-coupled rubbery polymer from the polymer cement also “kills” anyremaining living polymer by inactivating lithium end groups. After theasymmetrically tin-coupled rubbery polymer is recovered from thesolution, steam-stripping can be employed to reduce the level ofvolatile organic compounds in the asymmetrically tin-coupled rubberypolymer.

The asymmetrical tin-coupled rubbery polymers of this invention arecomprised of a tin atom having at least three polydiene arms covalentlybonded thereto. At least one of the polydiene arms bonded to the tinatom has a number average molecular weight of less than about 40,000 andat least one of the polydiene arms bonded to the tin atom has a numberaverage molecular weight of at least about 80,000. The ratio of theweight average molecular weight to the number average molecular weightof the asymmetrical tin-coupled rubbery polymer will also be within therange of about 2 to about 2.5.

The asymmetrical tin-coupled rubbery polymers of this invention are ofthe structural formula:

wherein R¹, R², R³ and R⁴ can be the same or different and are selectedfrom the group consisting of alkyl groups and polydiene arms (polydienerubber chains), with the proviso that at least three members selectedfrom the group consisting of R¹, R², R³ and R⁴ are polydiene arms, withthe proviso that at least one member selected from the group consistingof R¹, R², R³ and R⁴ is a low molecular weight polydiene arm having anumber average molecular weight of less than about 40,000, with theproviso that at least one member selected from the group consisting ofR¹, R², R³ and R⁴ is a high molecular weight polydiene arm having anumber average molecular weight of greater than about 80,000, and withthe proviso that the ratio of the weight average molecular weight to thenumber average molecular weight of the asymmetrical tin-coupled rubberypolymer is within the range of about 2 to about 2.5. It should be notedthat R¹, R², R³ and R⁴ can be alkyl groups because it is possible forthe tin halide coupling agent to react directly with alkyl lithiumcompounds which are used as the polymerization initiator.

In most cases, four polydiene arms will be covalently bonded to the tinatom in the asymmetrical tin-coupled rubbery polymer. In such cases, R¹,R², R³ and R⁴ will all be polydiene arms. The asymmetrical tin-coupledrubbery polymer will often contain a polydiene arm of intermediatemolecular weight as well as the low molecular weight arm and the highmolecular weight arm. Such intermediate molecular weight arms will havea molecular weight which is within the range of about 45,000 to about75,000. It is normally preferred for the low molecular polydiene arm tohave a molecular weight of less than about 30,000 with it being mostpreferred for the low molecular weight arm to have a molecular weight ofless than about 25,000. It is normally preferred for the high molecularpolydiene arm to have a molecular weight of greater than about 90,000with it being most preferred for the high molecular weight arm to have amolecular weight of greater than about 100,000.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

Benefits with tin-coupled IBRs, as compared to the linear IBRs, aredemonstrated by the following examples. These benefits include:

(1) Improvements in processability, particularly extrudability/extrudatequality.

(2) Treadwear improvement and rolling resistance reduction due toimproved carbon black dispersion with the tin-coupled IBR. Gooddispersion of carbon black prevents carbon particles from forming anetwork of carbon black in the vulcanizate and reduces hysteresisresulting from carbon black aggregates. This is known as Payne effect.The higher the Payne effect, the better the carbon black dispersion. ThePayne effect can be measured as follows:${{Payne}\quad {effect}} = {\frac{G^{\prime}\quad {at}\quad 10\% \quad {strain}}{G^{\prime}\quad {at}\quad 1\% \quad {strain}} \times 100}$

EXAMPLE 1

In this example, a coupled isoprene-butadiene rubber (IBR) was preparedin a one-gallon (3.8 liters) batch reactor at 70° C. In the procedureused, 2,000 grams of a silica/molecular sieve/aluminum dried premixcontaining 19.0 weight percent of a mixture of isoprene and1,3-butadiene in hexanes at the ratio of 10:90 was charged into aone-gallon (3.8 liters) reactor. After the amount of impurity in thepremix was determined, 4.0 ml of a 1.0 M solution of n-butyl lithium (inhexane) was added to the reactor. The target Mn (number averagedmolecular weight) was 100,000. The polymerization was allowed to proceedat 70° C. for three hours. An analysis of the residual monomer indicatedthat monomers was all consumed. Then, 1.0 ml of a 1 M solution of tintetrachloride (in hexane) was added to the reactor and the couplingreaction was carried out at the same temperature for 30 minutes. At thistime, 1.5 phr (parts per 100 parts by weight of rubber) of4-t-butylcatechol and 0.5 phr of TMEDA was added to the reactor toshortstop the polymerization and to stabilize the polymer.

After the hexane solvent was evaporated, the resulting SIBR was dried ina vacuum oven at 50° C. The coupled IBR was determined to have a glasstransition temperature (Tg) at −95° C. It was also determined to have amicrostructure which contained 7 percent 1,2-polybutadiene units, 87percent 1,4-polybutadiene units, 1 percent 3,4-polyisoprene units and 9percent 1,4-polyisoprene units. The Mooney viscosity (ML-4) of thecoupled IBR made was determined to be 99.

EXAMPLES 2-4

The procedure described in Example 1 was utilized in these examplesexcept that the isoprene to 1,3-butadiene ratio were changed from 10:90to 15:85, 20:80 and 30:70. The Tgs, Mooney viscosities (ML-4) andmicrostructures of these tin-coupled IBRs are listed in Table I. The30/70 IBR (Example 4) was determined to have an Mn (number averagedmolecular weight) of 386,000 and a Mw (weight averaged molecular weight)of 430,000. The precursor of Example 4 (ie, base polymer prior tocoupling) was also determined to have an Mn of 99,000 and an Mw of112,000.

TABLE I Isoprene/Bd Tg Microstructure (%) Ex. Composition (° C.) ML-41,2-PBd 1,4-PBd 3,4-PI 1,4-PI 1 10/90 −95 99 7 83 2  8 2 15/85 −93 91 877 1 14 3 20/80 −90 82 8 72 1 19 4 30/70 −87 84 7 63 3 27

EXAMPLES 5-8

In these examples, linear IBRs were prepared in a one-gallon reactor.The procedure described in Example 1 was utilized in these examplesexcept that no coupling agent (tin tetrachloride) was used in theseexperiments and the target Mn was changed to 300,000 from 100,000. Theisoprene to 1,3-butadiene ratios were 10:90, 15:85, 20:80 and 30:70. TheTgs, Mooney viscosities (ML-4), Mns (number averaged molecular weights),Mws (weight averaged molecular weights) and microstructures of theselinear IBRs are listed in Table II.

TABLE II Isoprene/Bd Tg Microstructure (%) Ex. Composition (° C.) ML-4Mn Mw 1,2-PBd 1,4-PBd 3,4-PI 1,4-PI 5 10/90 −96 88 308K 326K 7 83 1  9 615/85 −94 81 307K 329K 7 77 1 15 7 20/80 −92 82 317K 338K 7 72 1 20 830/70 −89 87 313K 332K 6 62 2 30

EXAMPLE 9

The tin-coupled IBR prepared in this experiment was synthesized in athree-reactor (10 gallons each) continuous system at 90° C. A premixcontaining isoprene and 1,3-butadiene in hexane was charged into thefirst reactor continuously at a rate of 65.6 grams/minute. The premixmonomer solution containing a ratio of isoprene to 1,3-butadiene of30:70 and had a total monomer concentration of 14 percent.Polymerization was initiated by adding 0.128 M solution of n-butyllithium into the first reactor at a rate of 0.4 grams per minute. Mostof the monomers were exhausted at the end of the second reactor and theresulted polymerization medium containing the live ends was continuouslypushed into the third reactor where the coupling agent, tintetrachloride, (0.025 M solution in hexane) was added at a rate of 0.34grams per minute. The residence time for all three reactors was set at1.5 hours to achieve complete monomer conversion in the second reactorand complete coupling in the third reactor. The polymerization mediumwas then continuously pushed over to a holding tank containing the TMEDAand an antioxidant. The resulting polymer cement was then steam-strippedand the recovered IBR was dried in an oven at 60° C. The polymer wasdetermined to have a glass transition temperature at −85° C. and have aMooney ML-4 viscosity of 90. It was also determined to have amicrostructure which contained 8 percent 1,2-polybutadiene units, 60percent 1,4-polybutadiene units, 29 percent 1,4-polyisoprene units and 3percent 3,4-polyisoprene units. The polymer was determined to have a Mn(number averaged molecular weight) of 185,000 and a Mw (weight averagedmolecular weight) of 276,000. The precursor of this polymer (i.e., basepolymer prior to coupling) was also determined to have an Mn of 88,000and an Mw of 151,000.

Unlike the Example 4 (prepared and coupled in a batch process) whichshowed a symmetrical coupling of four linear precursor polymers, thepolymer produced in this example via the continuous process hadunsymmetrical coupling base on GPC molecular data shown above.

EXAMPLE 10-12

The isoprene-butadiene rubbers made in Example 4, 8 and 9 were thencompounded utilizing a standard tire tread test formulation. The tiretread test formulations were made by mixing 100 parts of rubber beingtested with 50 parts of carbon black, 5 parts of processing oil, 2 partsof stearic acid, 3 parts of zinc oxide, 1 part of microcrystalline wax,0.5 part of paraffin wax, 1 part of a mixed aryl-p-phenylenediamineantioxidant, 2 parts of N-(1,3-dimethyl butyl)-N′-phenyl-p-phenylenediene and 1.4 parts of sulfur. The physical properties of the compoundedtire tread formulations are reported in Table III.

TABLE III Example 8 4 9 Rubber Type Linear Batch coupled continuouscoupled Rheometer, 150° C. ML, dNm 1.21 1.49 1.67 MH, dMm 24.06 25.7123.71 ts1, min 4.06 4.86 5.39 T25, min 5.62 5.78 6.50 T90, min 10.369.71 9.88 Autovibron, 11 Hz tan delta at 60° C. 0.113 0.083 0.072 G′ at10% 2.494 2.294 2.195 G° at 1% 3.435 2.732 2.566 Payne effect 72.6 84.485.5

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A process for preparing an asymmetricaltin-coupled rubbery polymer which comprises: (1) continuouslypolymerizing at least one diene monomer selected from the groupconsisting of 1,3-butadiene and isoprene to a conversion of at leastabout 90 percent utilizing an anionic initiator to produce a polymercement containing living polydiene rubber wherein the living polydienerubber chains are selected from the group consisting ofisoprene-butadiene chains, isoprene chains, and butadiene chains,wherein some of the living polydiene rubber chains are low molecularweight polydiene rubber chains having a number average molecular weightof less than about 40,000, and wherein some of the living polydienerubber chains are high molecular weight polydiene rubber chains having anumber average molecular weight of greater than about 80,000; and (2)continuously adding a tin halide to the polymer cement in a separatereaction vessel to produce the asymmetrically tin-coupled rubberypolymer, wherein said asymmetrical tin-coupled rubbery polymer has apolydispersity which is within the range of about 2 to about 2.5.
 2. Aprocess as specified in claim 1 wherein the anionic initiator is anorganolithium compound.
 3. A process as specified in claim 2 wherein thetin halide is a tin tetrahalide.
 4. A process as specified in claim 2wherein the tin halide is tin tetrachloride.
 5. A process as specifiedin claim 4 wherein the polymer cement further contains intermediatemolecular weight living polydiene rubber chains having a number averagemolecular weight which is within the range of about 45,000 to about75,000.
 6. A process as specified in claim 5 wherein the low molecularweight polydiene chains have a number average molecular weight of lessthan about 30,000, and wherein the high molecular weight polydienechains have a number average molecular weight of greater than about90,000.
 7. A process as specified in claim 3 wherein the livingpolydiene chains are isoprene-butadiene arms.
 8. A process as specifiedin claim 7 wherein the low molecular weight polydiene rubber chains havea number average molecular weight of less than 30,000.
 9. A process asspecified in claim 8 wherein the high molecular weight polydiene rubberchains have a number average molecular weight of greater than about90,000.
 10. A process as specified in claim 9 wherein the anionicinitiator is present at a level, which is within the range of 0.01 phmto 0.1 phm.
 11. A process as specified in claim 10 wherein thepolymerization is conducted at a temperature, which is within the rangeof about 30° C. to about 125° C.
 12. A process as specified in claim 11wherein the high molecular weight polydiene rubber chains have a numberaverage molecular weight of greater than about 100,000.
 13. A process asspecified in claim 12 wherein the anionic initiator is present at alevel, which is within the range of 0.025 phm to 0.07 phm.
 14. A processas specified in claim 13 wherein the polymerization is conducted at atemperature, which is within the range of about 60° C. to about 85° C.